1. Field of the Invention
This invention relates to the field of semiconductor manufacturing and more particularly, to a structure, apparatus, and method for handling semiconductor wafers in a manner which will permit transporting the wafer from one processing station to another without touching the surface of the wafer, thus reducing formation of loose particulates and permitting processing of both the top and bottom surfaces of the wafer.
2. Description of the Relevant Art
Semiconductor wafers, such as silicon wafers, are becoming increasingly large in terms of diameter. Larger diameter wafers permit placement of more devices, such as chips or dies, on the wafer thereby increasing the potential yield of the wafer. Large diameter wafers, such as those with diameters of 450 mm or greater, are increasingly difficult to handle, either manually or by robotic means. Wafers need to be physically moved to a number of processing stations for a number of various processes to be performed on the wafer. The difficulty of handling larger wafers coupled with the number of times a wafer is moved increases the risk of breakage or other damage to the wafer and its surface.
Handling of smaller diameter wafers is conventionally accomplished by using various wafer transfer mechanisms (WTMs) such as specialized tweezers for grasping the edge of the wafer and vacuum wands for applying suction to the surface of the wafer. Processing technicians use such WTMs on smaller wafers and manually move the wafers to the various processing stations. Using conventional WTMs, such as vacuum wands or tweezers, on large diameter wafers can more easily result in the wafer being damaged or broken. WTMs also tend to introduce contaminates onto the surface of the wafer as well as the dislodgment of particulates from the wafer surface. Such contamination and dislodgment can lead to failure of some of the chips or dies located on the wafer, thus lowering the yield of chips per wafer. In addition, large diameter wafers contain more chips or dies than smaller diameter wafers so that the cost of dropped or otherwise destroyed wafer due to handling is increased.
In addition, large diameter wafers are heavier than small diameter wafers. This increased weight makes it more difficult to transport the large diameter wafer using conventional WTMs and vacuum wands. Mechanical WTMs, such as tweezers, must have more force applied to prevent the wafer from dropping. This increased pressure can also contribute to damaging the chips or dies located on the wafer by introducing contaminates onto the wafer surface or dislodging particulates from the wafer surface.
In a conventional semiconductor process, integrated circuit structures, such as chips or dies, are usually only formed on one surface of the semiconductor wafer. There is increased interest, however, in processing both sides of the wafer. For example, it is sometimes necessary to remove oxide films from the backside of wafers without the surface contact of either the top or backside of the wafer. To remove oxide films that have been deposited, the front side of the wafer is conventionally pushed upward to a grounding surface, thereby endangering the already processed surface with a contacting and rubbing motion which creates particulate.
Conventional wafer processing steps are generally performed by resting the backside of the wafer on a flat support surface. In order to process the backside of the wafer, conventional means may require turning the wafer over and resting it on the front side of the wafer. This also involves resting the wafer on its front side which increases the likelihood that contaminates will be introduced to the chips or dies on the front side of the wafer. This involves increased processing steps (turning the wafer over) as well as more direct handling of the wafer. Increased direct handling adds further risk of damaging the chips or dies contained on the wafer from particulates that may become dislodged from such handling.
Chemical vapor deposition (CVD) is a processing step well known in the field of semiconductor fabrication. One problem with CVD fabrication is distributing the deposited film evenly over the wafer surface. Using conventional fabrication means, some parts of the wafer tend to receive a thicker film while other areas receive a thinner film Uneven wafer surfaces are undesirable because they tend to make subsequent processing more difficult and more prone to errors. Uneven wafer surfaces can effect photolithographic Processes and may cause notched interconnects which are susceptible to failure or reduced lifetime causing the device to fail after it has been sold to a customer. Because these types of defects may not manifest themselves until sometime well after fabrication, these types of reliability problems are very difficult to detect using conventional testing means.
Depositing films uniformly and evenly onto the surface of wafers through CVD processing is increasingly difficult on larger diameter wafers because of the inherent difficulty in maintaining a uniform deposition rate across the entire wafer. Cross-sections of wafers after CVD processing often appear wavy. As the wafer is subjected to further CVD processes, the wavy effect may become further and further pronounced. Eventually, mechanical grinding processes may need to be employed to reduce film thickness variations. Mechanical grinding processes are exceptional processes that add further risk of damage to the surface of the wafer. Not only can these processes introduce contaminates and loosen particulates on the wafer surface, but if not done properly these processes can grind into and damage the devices, such as chips and dies, which have been positioned on the wafer.
As complexity of integrated circuit devices continues to increase, circuit designers continue to add layers of circuitry to the wafers surface. Adding more layers of circuitry requires more CVD processing which can lead to undesirable effects as explained above. In addition, more overall steps and processes need to be performed to create more complex devices. As a result, more of an investment, in terms of resources and time, is made on the wafer surface. Therefore it is increasingly important to reduce the risk of contamination and formation of loose particulate as well as increase the quality of the wafer surface in terms of uniformity on larger diameter wafers.
As is described in further detail below, the prior art describes handling wafers by their edges or laying wafers flat against a platform. The prior art does not address the problem of both avoiding touching of the surface of the wafer and the need to have a handling mechanism which is independent of the wafer diameter.
Murdoch U.S. Pat. No. 5,046,909 describes a wafer holding assembly wherein a wafer is handled by a retaining ring which encircles the wafer and protruding wafer-engaging clips with ceramic tips which engage the edge of the wafer as well as a robotics system for loading a plurality of wafers into the wafer handling retaining ring. Murdoch does not, however, address the problem associated with increasing the wafer diameter without replacing or modifying the handling equipment. In addition, it is possible that holding large diameter wafers by the wafers' edges may be prone to slippage and accidental drops due to the increased size and weight of wafers roughly 450 millimeters in diameter.
Dean et all U.S. Pat. No. 4,473,455 describes a wafer holding assembly wherein a number of wafers are loaded onto a plate. Pedestal elements engage the backside surfaces of the wafers. The wafers are held in the plate by clips which are attached to springs mounted on the plate and which engage the edge of the front surface of the wafers. Dean does not, however, address the problem of touching the surface of the wafer. In Dean, wafers are rested on a pedestal from the backside of the wafer which can loosen particulates and contaminate the surface of the wafer. In addition, Dean does not address processing of the backside of the wafer.
Shaw U.S. Pat. Nos. 4,306,731 and 4,779,877 describe a wafer support assembly comprising a wafer plate assembly having an aperture larger than the diameter of the wafer. Spring clips comprising spring bands or spring wires carried by the wafer plate assembly have arcuate ends which engage the wafer surfaces adjacent the edges of the wafer. Again, Shaw does not address the problem of touching the surface of the wafer which can loosen particulate and cause contamination of the wafer surface.
It would be highly desirable to provide a wafer, process, and apparatus which would be independent of the diameter of the wafer surface unlike many methods and apparatus in the prior art. In addition, it would be desirable to provide a wafer, process, and apparatus which would permit the transport of the wafer from one process station to another without directly touching the surface of the wafer. It would also be desirable to permit transport of the wafer in a manner which would allow processing of both sides of the wafer without increased contact and handling of the wafer surface.